1. Field of the invention
The present invention relates generally to flight recording systems for helicopters and relates more specifically to a system for automatically recording the flying hours of helicopter rotor blades.
2. Discussion of the prior art
Aircraft manufacturers have established various service life limits for the rotating parts of aircraft based primarily on the number of repeated and/or alternating fatigue causing stress cycles undergone by the rotating parts. The manufacturers have generally defined a cycle as a flight consisting of the usual start, takeoff, landing and shutdown. Various less usual events have been given a weight as a full cycle or a fraction of a cycle.
Presently, these stress cycles are kept track of by log entries by the pilot or copilot. Generally, also the records are not kept separately for each unit in an aircraft, resulting in inaccurate records and unnecessary replacements. This is particularly pertinent in the life of helicopter rotor blades since the fatigue factors are not of any concern while the engines are merely idling and the rotor blades are not revolving at lifting speeds.
If a system could be found which would automatically keep track of stress cycles, a great deal of accuracy would result. Several prior art attempts have been made such as, for example, U.S. Pat. No. 3,938,762 to Murphy which discloses a rotor blade force track sensing system and automatic span tracking system comprising a vibration sensor mounted to sense manifestation of rotor unbalance causing vibration of the mast support structure. As the mast rotates, unbalanced forces create a sinusoidal vibration in the support structure at the location of the sensor of frequency equal to the speed of shaft rotation. A radial force is sensed when the resultant of the unbalance forces coincides with a reference direction. A radial force in the opposite direction is sensed when the resultant has rotated 180 degrees from the reference direction. The vibration sensor and associated detector circuitry provides a signal proportional to their sinusoidal reaction force or force track error.
U.S. Pat. No. 3,565,206 to Managhan discloses a safety device for unauthorized use of automotive vehicles consisting of a hard S-shaped spring having one end secured to the rotating shaft of the speed indicating arm of a speedometer. The free end of the S-shaped spring is provided with a U-shaped member adapted to engage a roller that is located on the outer end of the arm of an electric switch that is connected into the electric ignition system of an automotive vehicle. The engagement of the outer end of the S-shaped spring takes place when the vehicle has reached a predetermined speed and the shaft of the speedometer has rotated to a predetermined position. A spring-loaded pushbutton is used to activate a swing arm that will reset the safety device by pressing against the S-shaped spring.
U.S. Pat. No. 3,954,229 to Wilson discloses an automatic one-per-rev control system comprising means for generating a reference signal having a frequency related to the speed of rotation of the helicopter rotor and at a phase corresponding to a desired cyclic pitch. In addition, the system includes means for generating a control voltage having a magnitude related to the one-per-rev vibrations in the fuselage of a helicopter. The control voltage is then applied to a modulator wherein it is modulated by the reference signal and applied to the helicopter rotor cyclic pitch control system.
U.S. Pat. No. 3,979,579 to Kleinpeter discloses an aircraft engine cycle recorder which senses engine starting, engine shutdown, landing gear status, engine reversal and throttle setting and derives unit and fractional cycles. The system includes a first memory responsive to the sensors for producing an indication of when the aircraft is in flight and a second memory for producing an indication of when the landing gear are dropped in flight. These memory indications are fed to gates which index the units counter when a takeoff, landing and engine shutdown cycle is completed and which index the fractions counter when a re-takeoff without engine shutdown is accomplished. Other gates cause the units counter to be indexed when the engine is restarted in flight.
Each of the prior art references disclose the use of switch sensors controlling indicators of one type or the other. Neither of the prior art references show the use of a speed related rotor switch nor the recordation of the hours of operation.